The growing use of micro or millimetre frequencies, especially within wireless communications requires low-loss, high Q passive components. One important aspect is the fabrication process of these components, which must be inexpensive and allow batch processing.
Filters, for example, are one of the most important components. The prior art cavity filters or other microwave or millimetre wave elements made in micromachined technique appear as they were milled in metal, i.e. they have perpendicular angles in quadratic and square shaped “boxes”. These are simple to compute and easy to etch, for example, if low productive dry etching method is used.
It is known to etch a [110] silicon substrate by means of fast wet etching methods; however, they follow crystalline structure, which usually are approximately 60 degrees. Some cheap sensors for air bags, for example, are designed in geometrical shapes that suite wet etching. These sensors do not use radio frequencies or cavities but through vibrations they sense motion and they are produced by etching off silicon volumes.
Wet etching is a blanket name that covers the removal of material by immersing the wafer in a liquid bath of the chemical etchant. Wet etchants fall into two broad categories: isotropic etchants and anisotropic or preferential etchants.
Isotropic etchants attack the material being etched at the same rate in all directions. Anisotropic etchants attack the silicon wafer at different rates in different directions, and so there is more control of the shapes produced. Some etchants attack silicon at different rates depending on the concentration of the impurities in the silicon (concentration dependent etching).
Micromachined filters in which the cavities are attached to a metallic layer and the “cap” of the filter having slot connection made through conventional circuit board manufacturing technique are described in “A high performance K-Band diplexer using high-Q micromachined cavities”, Michael J. Hill et al, department of Electrical and Computer Engineering, University of Arizona, Tucson, Ariz. 85721-0104. According to this paper, which is directed at microwave diplexers two high Q cavity resonators, a Duroid-based high performance diplexer has been designed, fabricated and measured. This diplexer shows transmit/receive bandwidths of 2.39% and 1.8% and insertion losses of 2.38 dB and 2.89 dB, respectively. Channel centre frequencies of 18.8 GHz and 20.7 GHz provide a channel separation of approximately 9% and channel-to-channel isolation greater than 24 dB. Utilizing machined aluminium cavities and a Duroid substrate the diplexer design provides insight into cavity based diplexer construction, allowing for the design of a silicon based micromachined cavity diplexer. Simulation results from this silicon-based diplexer are also presented. One disadvantage with machined filters in Duroid-based technique is not being suitable for low cost batch production. In addition large tolerances do not allow fabrication of filters with desired performances.
Cavities having inclined walls are known through “A Finite Ground Coplanar Line-To-Silicon Micromachined Waveguide Transition”, James P. Becker et al, IEEE Transactions on Microwave Theory and Techniques, Vol. 49, No. 10 October 2001. A channel is etched through a wet anisotropic etching. The channel has a triangular cross-section. Thus, this document concerns another planar etching technique, intended for high frequency applications.